Arianna Occhipinti is another must-try. She is the daughter of one of the guys from COS, and makes the best frappato about.
I like “natural” - and indeed COS and Occhipinti are on that spectrum - but Cornelissen is just too out there, even for me.
Considering the food in Sicily is to my mind the best in the world, I’ve never had much luck eating in Palermo, but I would advise you to eat in the new(er!) part of town on the other side of the theatre. Most places I’ve found in the old city are tourist traps (maybe it’s just my bad luck?)
I don’t know the Catania/Siracusa area much.
I would recommend trips out from Palermo to visit Trapani, Marsala, and Selinunte (the best ruin on the island in my book). Of course you need a car for this. Marsala has the wonderful enoteca “La Sirena Ubriaca”, with some fabulous local wines matched with brilliant tapa-style nibbles. The Grillos are an eye opener. Marco De Bartoli makes great wine, and Nino Barraco’s “natural” grillo and zibibbo are another must-try.